More AC - 90 CQ

Hi List,
As long as we're on AC problems here, I've got a 1990 CQ with the AC
Delco-style automatic climate controls.
The problem I'm having is that the AC cycles on and off at about a 45
second period. I set the controls to auto/LO/fan high and rev'd the
motor. I can see the clutch on the AC compressor coming on and off at
the 45-second interval. It's very annoying while driving because the car
lurches a little every time the compressor comes back.
I'm hoping it's just low on coolant and I can find some R12, but does
anyone have any other ideas? I still haven't found a Bentley for this
car, so I don't have the factory service checklist for this AC system.
Thanks!
-m